1. Technical Field
The present invention relates to oscillator circuits for piezoelectric vibrators, and more particularly, to an oscillator circuit suitable for causing a piezoelectric vibrator immersed in liquid to oscillate, an oscillator circuit adjusting method, and a mass measuring apparatus using the oscillator circuit.
2. Background Art
In recent years, quartz crystal microbalances (QCMs) using a quartz vibrator, which is a piezoelectric vibrator, have been receiving attention. QCMs utilize the fact that deposition of a substance on an electrode of a quartz vibrator reduces the oscillation frequency. Since QCMs are capable of detecting masses on the order of nanograms (ng) or less, QCMs are applied, as biosensors, chemical sensors, and the like, to detect micro materials in a wide range of fields, such as medical, biochemical, food, and environmental measurement.
For example, in some cases, quartz vibrators immersed in liquid are used as mass measuring apparatuses. The effective crystal impedance (hereinafter, referred to as a “CI value”) of quartz vibrators in air is very different from that of quartz vibrators in liquid; the CI value in liquid is about ten to thirty times larger than the CI value in air. Since the increase in the CI value makes oscillation of quartz vibrators more difficult, it is difficult to cause quartz vibrators in liquid to oscillate with the same circuit conditions for causing quartz vibrators in air to oscillate. Thus, oscillation of quartz vibrators in liquid has been made possible by increasing the amplification of an oscillator circuit (for example, Japanese Unexamined Patent Application Publication No. 11-163633 (Paragraph No. 0004 and FIG. 1)). Also, an oscillator circuit that causes a plurality of quartz vibrators having different fundamental frequencies to oscillate by the same circuit by using inverters made of high speed CMOS devices has been proposed (Japanese Unexamined Patent Application Publication No. (Paragraph No. 0015)). In Patent Document 2001-289765, however, the operation only in a gas phase is suggested.
Since an oscillator circuit is a feedback circuit that forms an oscillator loop, if oscillation conditions of the oscillator loop are satisfied, oscillation may occur but not by way of a vibrator, or even in the case where the oscillation does occur by way of the vibrator, the oscillation may occur at a frequency that is not the resonance frequency of the vibrator. Thus, if the amplification is increased, as in Patent Document 11-163633, such undesirable oscillation is likely to occur. Also, in a case where the oscillator circuit is incorporated in the actual circuit, the oscillator circuit is electrically connected to various other circuits. Thus, if the amplification of the oscillator circuit is increased, oscillation may occur in the various other circuits that are not provided with vibrators. Therefore, it is difficult to achieve stable oscillation merely by increasing the amplification.
FIG. 20 shows the variation in the CI value of a piezoelectric vibrator in air with respect to frequency and the variation in the CI value of a piezoelectric vibrator in liquid with respect to frequency. In air, the CI value of the piezoelectric vibrator that oscillates at a frequency of 148.25 MHz is about 20Ω. In contrast, in liquid, the CI value of the piezoelectric vibrator that oscillates at a frequency of 148.25 MHz is about 300Ω. Thus, it is difficult to cause the piezoelectric vibrator to oscillate in liquid by using the circuit that oscillates in air, as in the technology of Patent Document 2001-289765.
FIG. 21 shows the variation in phase of a piezoelectric vibrator in air with respect to frequency and the variation in phase of a piezoelectric vibrator in liquid with respect to frequency. In air, the phase of the piezoelectric vibrator is abruptly changed from −90 degrees to +90 degrees at a frequency near 148.25 MHz. In contrast, in liquid, the phase of the piezoelectric vibrator is only changed between about −90 degrees and about −50 degrees, not only at a frequency near 148.25 MHz, but also in a frequency range between 147.8 MHz and 148.6 MHz. Accordingly, the phase does not abruptly change. Thus, in Patent Document 2001-289765, an amplifier includes an inverter and the phase at the output side of the amplifier is always 180 degrees different from the phase at the input side of the amplifier. In other words, the phase at the output side of the amplifier cannot be equal to the phase at the input side of the amplifier.
In order to solve the problems described above, an object of the present invention is to ensure oscillation both in air and liquid.
Also, another object of the present invention is to ensure prevention of undesirable oscillation, such as parasitic oscillation, spurious oscillation, and feedback oscillation.